Trans fat has been perhaps the country's No. 1 food villain for more than a decade now - a target of state and regional bans, roundly rejected by nutrition and public health experts.

But it still haunts our food supply.

The artificial fat - an additive demonized even more widely than high fructose corn syrup and salt - lurks in hundreds of the most popular processed foods in the United States, from cookies and pies to potato chips and crackers. It's in frozen pizzas and Girl Scout cookies, saltine crackers and premade pie crusts.

It may even still hide in restaurant foods in California, despite a statewide ban that's been in place for four years. Because here's the catch: Under federal regulations, foods can be sold and marketed as trans-fat-free as long as there are fewer than 0.6 grams of the additive per serving.

A study released last week by the U.S. Centers for Disease Control and Prevention found that about 9 percent of the country's packaged food supply contains trans fat, which comes primarily from partially hydrogenated oils. But 85 percent of those products stated plainly on the packaging that they contained no trans fat at all.

"It's the same thing we always say: buyer beware," said Jo Ann Hattner, a San Francisco registered dietitian who teaches nutrition courses at Stanford.

The packaging may be misleading, Hattner said, but it's legal due to the loophole in federal policy. To confirm whether a food really is trans-fat-free, consumers may have to put on their reading glasses and check out the small print on the ingredient list.

If the words "partially hydrogenated" appear, there's trans fat in the box. In restaurants, customers can ask what products are used in the kitchen and specifically what kinds of fats the chefs are cooking with.

There's movement, at least, that makes nutritionists like Hattner hopeful. Last November, the Food and Drug Administration announced a tentative decision to mark trans fat as unsafe for consumption. If finalized, trans fats would be removed almost entirely from the national food supply. After four months of taking public input - 1,500 comments in all - the FDA is reviewing scientific and legal data before making a final decision.

"Partially hydrogenated oils are really poisons. There's not a safe level to have in the food," said Dr. Junaid Khan, a cardiothoracic surgeon at Alta Bates Summit Medical Center in Berkeley. "That's what the FDA is going to have to address."

Trans fat has been repeatedly linked to a variety of health problems, but primarily to worsening cholesterol levels. Studies have found that even low levels of trans fat - just 2 to 5 grams a day - can have detrimental effects.

The fat mostly comes from a manufacturing process in which hydrogen is added to oils to make them solid, thus making them more stable in foods that are likely to sit on grocery shelves or kitchen cupboards for many weeks or months - long after natural fats would have spoiled.

It became widely used in American processed foods about a century ago, and an even bigger push came in the '50s and '60s, when doctors and public health experts rallied against the saturated fats found primarily in meat and dairy products. That drove consumer interest in manufactured alternatives like margarine that were made up of trans fat.

It wasn't until 1990 that the first studies came out that suggested trans fat was no better than saturated fat - and, in fact, that it was probably worse. Part of the lack of concern for trans fats came from early misunderstandings of cholesterol, and the simplistic view that high cholesterol was bad and low was good. Early studies showed saturated fat raised cholesterol, but trans fat seemed to have a neutral effect.

But as doctors developed a more nuanced understanding of cholesterol, separating the "good" HDL cholesterol from the "bad" LDL cholesterol, they also better understood what was happening with trans fat: It increased the bad cholesterol and decreased the good stuff.

"They're like cigarettes. They do nothing good for you. They're only bad for you," said Katie Ferraro, a registered dietitian and assistant clinical professor of nutrition at the UCSF School of Nursing.

"It just happens that the way the biochemistry works, these particular trans fatty acids in the liver get converted to LDL," said Dr. Eleanor Levin, a cardiologist with Kaiser Permanente Santa Clara. "That tends to increase in your blood, and then it gets caught on your arteries and you get heart disease."

Until the FDA forces trans fats out of the U.S. food supply, consumers will need to remain vigilant, which may be both more and less complicated than it sounds. Here's the single easiest way to truly remove trans fat from the diet: don't buy any processed foods and don't eat in restaurants, especially places that offer cheap, fast and convenient meals.

"We've been telling people to read the labels. But perhaps the healthiest way to eat is to eat foods that don't have any nutritional labels at all - only foods that are fresh, not processed," said Khan.

And that message goes far beyond trans fat, Hattner added. Foods with trans fat probably are high in salt or loaded with sugar, too. In fact, seeing the words "partially hydrogenated" on a label should come across as a flashing red light," she said.

"It's a caution," Hattner said. "It's an indicator that there are probably other ingredients in that food you probably don't need either."

In fact, said Ferraro and other nutrition experts, trans fat consumption already has fallen greatly in the United States since the first major warnings against it came out. From 2000 to 2009, levels of trans-fatty acids in the blood of white adults in the United States fell 58 percent, according to a CDC study.

Trans fat, even though it's still sneaking into American diets, should no longer be a primary target of healthy food proponents, Ferraro said.

"This is not a health crisis, that people are eating trans fats still," Ferraro said. "The bigger crisis is that people are still buying packaged and processed foods that have all kinds of bad stuff in them."